Coating to be applied onto coating objects such as car bodies is required to form coating film of excellent performance such as scratch resistance, acid resistance, stain resistance and finished appearance.
Conventionally, melamine crosslinking coating is generally used as paint for coating objects such as car bodies. Melamine crosslinking coating is a paint which contains hydroxyl-containing resin as the base resin and melamine resin as the crosslinking agent, and shows high crosslinking density when cured by heating and excels in coating film performance such as scratch resistance and finished appearance. The coating film formed from the paint, however, has a defect that the melamine crosslinkage is easily hydrolyzed by acid rain and hence the film is inferior in acid resistance.
As a paint capable of forming a coating film superior in both acid resistance and scratch resistance, for example, JP Hei 6 (1994)-220397A discloses a two package type urethane crosslinking paint composition comprising hydroxyl-containing acrylic resin, hydroxyl-containing oligoester and isocyanate prepolymer. The coating film formed of this paint excels in acid resistance because urethane-crosslinkage is difficult to be hydrolyzed, but has a problem that its scratch resistance is insufficient.
Also JP Hei 10 (1998)-272414A discloses a hard top coating film-forming method by 3-coat-2-bake system, as a method for forming a top coat multilayer coating film excelling in acid rain resistance, scratch resistance, pollen soiling resistance and the like, which comprises successively applying a coloring paint and a first clear paint, heating to simultaneously cure the two coating films, thereafter applying onto the coated surface a second clear paint containing a hydrolyzate of specific silicon compound, and heating and curing the same. According to this coating film-forming method, hard coating film can be obtained by suitably selecting the hydrolyzate of silicon compound, but the coating film has insufficient scratch resistance and weatherability.